The true nature of time continues to elude us. But whether it is a fundamental part of the cosmos or an illusion made in our minds has profound implications for our understanding of the universe

Physics 15 June 2022, By Richard Webb

WE ARE BORN; we live; at some point, we die. The notion that our existence is limited by time is fundamental to human experience.

We can’t fight it – and truth be told, we don’t know what we are fighting against. Time is a universal whose nature we all – and physicists especially – fail to grasp. But why is time so problematic?

“If we had a really good answer to that question,” says Astrid Eichhorn, a theoretical physicist at the University of Southern Denmark in Odense, “then it wouldn’t be so problematic.”

On a certain level, time is simple: it is what stops everything happening at once. That might seem flippant, but it is at least something people can agree on. “The causal order of things is really what time is all about,” says Eichhorn.

Viewed this way, the existence of time can be interpreted as a necessary precondition for the sort of universe where things lead to other things, among them intelligent life that can ask questions, such as “what is time?”.

Beyond that, time’s essence is mysterious. For instance, why can things only influence other things in one direction in time, but in multiple directions in the three dimensions of space.

Most physical theories, from Isaac Newton’s laws of motion to quantum mechanics, skirt such questions. In these theories, time is an “independent variable” against which other things change, but which can’t be changed by anything else. In that sense, time exists outside physics, like the beat of a metronome outside the universe to which everything inside it plays out.

Source: What is time? The mysterious essence of the fourth dimension | New Scientist